How do you define dungeon? If I were running this on tabletop, I’d consider the entire underdark a mega-dungeon.
The way you put that (which I completely agree with) makes me think of the whole "dungeon", dungeon crawl dynamic as something that doesn't necessarily apply to a modern cRPG. Tabletop? Maps need to be manageable by humans. The cRPGs we played in the 80s and 90s? Maps had to be manageable by computers in which memory was measured in the ones and tens of megabytes, if not less for the games that came on six 5.25" floppies.
A discreet map, a handful of maps of levels in a dungeon. Boom. Dungeon. But that constraint really hasn't applied in a long time, has it? Sure, some games very intentionally make multi-level subterranean architectural structures populated with baddies, but there's nothing requiring that, is there?
The Underdark is a great example as you mention. It's a sprawling area that in tabletop would be many, many pages of hex maps. It meets many of the standard dungeon criteria in terms of being subterranean and populated with treasure and baddies, but to its inhabitants, it's essentially just outdoors, and it's large enough to have architectural structures contained within it; it's not a piece of architecture itself, and it's not a single discreet cave system.
I think "Dungeon" might be a word that's meant very, very figuratively when we're not constrained by what humans or old computers are/were capable of handling the playable logistics of.
